1. The Field of the Invention
The present invention relates to semiconductor optical amplifiers. More particularly, the present invention relates to semiconductor optical amplifiers whose gain is independent of the polarization of the optical signal being amplified and to methods of manufacturing or fabricating semiconductor optical amplifiers with polarization independent gain.
2. Background and Relevant Art
Optical communication systems have several advantages over other types of telecommunications networks. Optical fibers are typically made from insulative materials and are therefore less susceptible to interference from electromagnetic sources. Optical fibers also have higher bandwidth capability. In addition, optical fibers are both smaller and lighter than metal cables.
As optical signals are transmitted through the optical fibers of the communication network, the optical signals gradually become weaker over distance. Thus, the optical signals need to be refreshed or strengthened before the signals become too weak to detect. Before the advent of optical amplifiers, regenerators were used to refresh or strengthen the weakened optical signals. Regenerators convert the optical signal to an electrical signal, clean the electric signal, and convert the electrical signal back to an optical signal for continued transmission in the optical communication network. Regenerators, however, can typically only amplify one channel or a single wavelength. Optical amplifiers are an improvement to regenerators because optical amplifiers can amplify light signals of multiple wavelengths simultaneously. One type of optical amplifier is a semiconductor optical amplifier (SOA).
At a basic level, an SOA has multiple layers formed from compound semiconductor materials that are grown on a semiconductor substrate. An SOA usually includes an active layer or region and two cladding layers. The active layer provides optical gain to a light signal and the cladding layers, together with the active layer or region, construct an optical waveguide. The facets of the SOA are formatted by cleaving the semiconductor wafer along a crystal plane, thus forming a mirror. An anti-reflective coating is often applied to the facets to decrease the facet reflection. When an optical signal is injected from the input facet, the light is amplified by the gain of the active layer.
In some semiconductor optical amplifiers, the active layer includes a pair of compound semiconductor layers to confine the electrons inside the active layer, but the semiconductor optical amplifier will still need an extra pair of cladding layers to confine the optical mode. These cladding layers thus form a separate confinement structure. Thus, the electrons and the optical mode are separately confined by different layers.
In some instances, a series of layers known as quantum wells serve as the active region of an SOA. A quantum well SOA has desirable characteristics such as high efficiency, low optical loss, high gain and low operation current. A disadvantage of quantum well SOAs is that the gain provide by either bulk material or the quantum well material is polarization dependent. Quantum well SOAs are thus inherently polarization sensitive.
Quantum well SOAs that are polarization sensitive have limited use because the optical signal is not only amplified but also changed. Polarization sensitivity results in one mode of the optical signal receiving a different gain than another mode of the optical signal. In addition, different modes of a polarized optical signal may travel at different speeds through the optical fiber. This makes the problem of polarization sensitive optical amplifiers more critical.
Attempts to limit or correct this problem have centered on including both compressively strained quantum wells and tensile strained quantum wells in the active region of the SOA. The assumption of including compressively strained quantum wells and tensile strained quantum wells is that the compressively strained quantum wells yield higher gain for transverse electric (TE) modes and the tensile strained quantum wells yield higher gain for transverse magnetic (TM) modes.
Current practice is to adjust the TE mode gain with respect to the TM mode gain through careful design of the composition of the grown material in the active region, the widths of the quantum wells, the amount of strain, and the number of layers included in the active region in order to reduce the polarization sensitivity of SOAs. Reducing or eliminating the polarization sensitivity of SOAs is difficult to achieve because of the uncertainties involved in material growth, where the interface and the quality of the bulk material can vary and therefore cause the imbalance between the TE and TM modal gain, resulting in polarization dependent gain.
These and other problems are overcome by the present invention which is directed to a semiconductor optical amplifier that has polarization independent gain such that the modes of an optical signal receive essentially the same gain. The gain of semiconductor optical amplifiers, including quantum well semiconductor optical amplifiers, is balanced by adding a polarization adjusting layer to the structure of a semiconductor optical amplifier. The polarization adjusting layer effectively repositions an optical mode within an active region of the semiconductor optical amplifier such that the TE mode gain and the TM mode gain of the optical signal are more equal and the polarization sensitivity of the semiconductor optical amplifier is reduced.
One example of an active region of a semiconductor optical amplifier includes both compressively strained quantum wells and tensile strained quantum wells. The compressively strained quantum wells contribute to TE mode gain of the optical signal while the tensile strained quantum wells contribute to the TM mode gain of the optical signal. After the active region has been formed or grown and the cladding layers have also been formed, a polarization adjusting layer is formed on the structure of the semiconductor optical amplifier.
The polarization adjusting layer changes the refractive index profile of the semiconductor optical amplifier in the vertical direction such that the optical mode (or optical spot) is repositioned within the active region. The spot of the optical signal propagating through the active layer can be adjusted in the vertical direction by changing a thickness of the polarization adjusting layer. This lessens the impact of the quantum well growth on the polarization dependence of the semiconductor optical amplifier.
The proper thickness of the polarization adjusting layer can be determined by etching the polarization adjusting layer to different thicknesses, for example, by dropping a slice of the wafer in an etching solution at a fixed speed. The gain of an optical signal is then measured to determine which thickness of the polarization-adjusting layer enables the semiconductor optical amplifier to be polarization independent.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.